As stated above, you will need to carry your own Employers & Public Liability even if you subcontract out all supply & installation work – even if only on a contingent liability basis (to protect yourself against a subcontractor’s own cover failing).
Collateral Warranties are a whole different ball-ache and, generally speaking are not directly insurable. A good solicitor who understands the industry can give advice although there are PI insurers out there that offer a ‘free’ checking service for their clients which will highlight any issue that would fall outside the scope of policy cover – AIG come to mind for this. It tends to be the CW that drives the need to carry PI for 12 years as this is the ‘normal’ period of them.